diff --git a/core/includes/common.inc b/core/includes/common.inc
index 8e0d53e..0c70f83 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -3338,6 +3338,7 @@ function drupal_render_page($page) {
  *
  * @see element_info()
  * @see _theme()
+ * @see themeable
  * @see drupal_process_states()
  * @see drupal_process_attached()
  */
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 12c11e6..ed25af9 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1849,10 +1849,9 @@ function template_preprocess_container(&$variables) {
  * This function is called for theme hooks implemented as templates only, not
  * for theme hooks implemented as functions. This preprocess function is the
  * first in the sequence of preprocessing functions that are called when
- * preparing variables for a template. See _theme() for more details about the
- * full sequence.
+ * preparing variables for a template.
  *
- * @see _theme()
+ * @see themeable
  */
 function template_preprocess(&$variables, $hook, $info) {
   // Tell all templates where they are located.
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 402c9f7..2656d4a 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -286,7 +286,7 @@ public function getBaseHook($hook) {
    * for base hooks (e.g., 'block__node' for the base hook 'block') need to be
    * determined based on the full registry and classified as 'base hook'.
    *
-   * @see _theme()
+   * @see themeable
    * @see hook_theme_registry_alter()
    *
    * @return \Drupal\Core\Utility\ThemeRegistry
diff --git a/core/modules/block/block.api.php b/core/modules/block/block.api.php
index 9210f82..6664ccc 100644
--- a/core/modules/block/block.api.php
+++ b/core/modules/block/block.api.php
@@ -20,8 +20,8 @@
  * If the module wishes to act on the rendered HTML of the block rather than
  * the structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement hook_preprocess_HOOK() for
- * block.html.twig. See drupal_render() and _theme() documentation respectively
- * for details.
+ * block.html.twig. See drupal_render() documentation or the
+ * @link themeable Default theme implementations topic @endlink for details.
  *
  * In addition to hook_block_view_alter(), which is called for all blocks, there
  * is hook_block_view_BASE_BLOCK_ID_alter(), which can be used to target a
diff --git a/core/modules/node/node.api.php b/core/modules/node/node.api.php
index 8293618..2c02948 100644
--- a/core/modules/node/node.api.php
+++ b/core/modules/node/node.api.php
@@ -817,8 +817,8 @@ function hook_node_view(array &$build, \Drupal\node\NodeInterface $node, \Drupal
  * If the module wishes to act on the rendered HTML of the node rather than the
  * structured content array, it may use this hook to add a #post_render
  * callback.  Alternatively, it could also implement hook_preprocess_HOOK() for
- * node.html.twig. See drupal_render() and _theme() documentation respectively
- * for details.
+ * node.html.twig. See drupal_render() documentation or the
+ * @link themeable Default theme implementations topic @endlink for details.
  *
  * @param &$build
  *   A renderable array representing the node content.
@@ -828,6 +828,7 @@ function hook_node_view(array &$build, \Drupal\node\NodeInterface $node, \Drupal
  *   The entity view display holding the display options configured for the node
  *   components.
  *
+ * @see themeable
  * @see node_view()
  * @see hook_entity_view_alter()
  *
diff --git a/core/modules/system/entity.api.php b/core/modules/system/entity.api.php
index 50b9f6a..10090bf 100644
--- a/core/modules/system/entity.api.php
+++ b/core/modules/system/entity.api.php
@@ -480,7 +480,7 @@ function hook_entity_view(array &$build, \Drupal\Core\Entity\EntityInterface $en
  * structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement hook_preprocess_HOOK() for
  * the particular entity type template, if there is one (e.g., node.html.twig).
- * See drupal_render() and _theme() for details.
+ * See drupal_render() for details.
  *
  * @param array &$build
  *   A renderable array representing the entity content.
@@ -490,6 +490,7 @@ function hook_entity_view(array &$build, \Drupal\Core\Entity\EntityInterface $en
  *   The entity view display holding the display options configured for the
  *   entity components.
  *
+ * @see themeable
  * @see hook_entity_view()
  * @see hook_comment_view_alter()
  * @see hook_node_view_alter()
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 4447052..d6c7e70 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -977,12 +977,8 @@ function hook_permission() {
 /**
  * Register a module or theme's theme implementations.
  *
- * The implementations declared by this hook have several purposes:
- * - They can specify how a particular render array is to be rendered as HTML.
- *   This is usually the case if the theme function is assigned to the render
- *   array's #theme property.
- * - They can return HTML for default calls to _theme().
- * - They can return HTML for calls to _theme() for a theme suggestion.
+ * The implementations declared by this hook specify how a particular render
+ * array is to be rendered as HTML.
  *
  * @param array $existing
  *   An array of existing implementations that may be used for override
@@ -1008,23 +1004,22 @@ function hook_permission() {
  *
  * @return array
  *   An associative array of information about theme implementations. The keys
- *   on the outer array are known as "theme hooks". For simple theme
- *   implementations for regular calls to _theme(), the theme hook is the first
- *   argument. For theme suggestions, instead of the array key being the base
- *   theme hook, the key is a theme suggestion name with the format
- *   'base_hook_name__sub_hook_name'. For render elements, the key is the
- *   machine name of the render element. The array values are themselves arrays
- *   containing information about the theme hook and its implementation. Each
- *   information array must contain either a 'variables' element (for _theme()
- *   calls) or a 'render element' element (for render elements), but not both.
+ *   on the outer array are known as "theme hooks". For theme suggestions,
+ *   instead of the array key being the base theme hook, the key is a theme
+ *   suggestion name with the format 'base_hook_name__sub_hook_name'.
+ *   For render elements, the key is the machine name of the render element.
+ *   The array values are themselves arrays containing information about the
+ *   theme hook and its implementation. Each information array must contain
+ *   either a 'variables' element (for using a #theme element) or a
+ *   'render element' element (for render elements), but not both.
  *   The following elements may be part of each information array:
- *   - variables: Used for _theme() call items only: an array of variables,
+ *   - variables: Only used for #theme in render array: an array of variables,
  *     where the array keys are the names of the variables, and the array
- *     values are the default values if they are not passed into _theme().
- *     Template implementations receive each array key as a variable in the
- *     template file (so they must be legal PHP/Twig variable names). Function
- *     implementations are passed the variables in a single $variables function
- *     argument.
+ *     values are the default values if they are not passed to #theme in render
+ *     array. Template implementations receive each array key as a variable in
+ *     the template file (so they must be legal PHP/Twig variable names).
+ *     Function implementations are passed the variables in a single $variables
+ *     function argument.
  *   - render element: Used for render element items only: the name of the
  *     renderable element or element tree to pass to the theme function. This
  *     name is used as the name of the variable that holds the renderable
@@ -1042,13 +1037,7 @@ function hook_permission() {
  *     this file; that extension will be added automatically by the default
  *     rendering engine (which is Twig). If 'path' above is specified, the
  *     template should also be in this path.
- *   - function: If specified, this will be the function name to invoke for
- *     this implementation. If neither 'template' nor 'function' is specified,
- *     a default function name will be assumed. For example, if a module
- *     registers the 'node' theme hook, 'theme_node' will be assigned to its
- *     function. If the chameleon theme registers the node hook, it will be
- *     assigned 'chameleon_node' as its function.
- *   - base hook: Used for _theme() suggestions only: the base theme hook name.
+ *   - base hook: Used for #theme suggestions only: the base theme hook name.
  *     Instead of this suggestion's implementation being used directly, the base
  *     hook will be invoked with this implementation as its first suggestion.
  *     The base hook's files will be included and the base hook's preprocess
@@ -1058,14 +1047,16 @@ function hook_permission() {
  *     suggestion may be used in place of this suggestion. If after
  *     hook_theme_suggestions_HOOK() this suggestion remains the first
  *     suggestion, then this suggestion's function or template will be used to
- *     generate the output for _theme().
+ *     generate the output for #theme in render array.
  *   - pattern: A regular expression pattern to be used to allow this theme
  *     implementation to have a dynamic name. The convention is to use __ to
  *     differentiate the dynamic portion of the theme. For example, to allow
  *     forums to be themed individually, the pattern might be: 'forum__'. Then,
  *     when the forum is themed, call:
  *     @code
- *     _theme(array('forum__' . $tid, 'forum'), $forum)
+ *     $render_array = array(
+ *       #theme => array('forum__' . $tid, 'forum'), $forum,
+ *     )
  *     @endcode
  *   - preprocess functions: A list of functions used to preprocess this data.
  *     Ordinarily this won't be used; it's automatically filled in. By default,
@@ -1085,6 +1076,7 @@ function hook_permission() {
  *   - theme path: (automatically derived) The directory path of the theme or
  *     module, so that it doesn't need to be looked up.
  *
+ * @see themeable
  * @see hook_theme_registry_alter()
  */
 function hook_theme($existing, $type, $theme, $path) {
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 50fa74e..1734a2e 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -107,7 +107,7 @@
  * suggestion alter hooks.
  *
  * @see drupal_render()
- * @see _theme()
+ * @see themeable
  * @see hook_theme()
  * @see hooks
  * @see callbacks
diff --git a/core/modules/taxonomy/taxonomy.api.php b/core/modules/taxonomy/taxonomy.api.php
index 9fa84c1..52d06e1 100644
--- a/core/modules/taxonomy/taxonomy.api.php
+++ b/core/modules/taxonomy/taxonomy.api.php
@@ -282,8 +282,9 @@ function hook_taxonomy_term_view(array &$build, \Drupal\taxonomy\Entity\Term $te
  * If the module wishes to act on the rendered HTML of the term rather than the
  * structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement
- * hook_preprocess_HOOK() for taxonomy-term.html.twig. See drupal_render() and
- * _theme() documentation respectively for details.
+ * hook_preprocess_HOOK() for taxonomy-term.html.twig. See drupal_render()
+ * documentation or @link themeable Default theme implementations topic @endlink
+ * for details.
  *
  * @param array &$build
  *   A renderable array representing the taxonomy term content.
diff --git a/core/modules/user/user.api.php b/core/modules/user/user.api.php
index ab6e2b6..6804e69 100644
--- a/core/modules/user/user.api.php
+++ b/core/modules/user/user.api.php
@@ -338,15 +338,15 @@ function hook_user_view(array &$build, \Drupal\user\UserInterface $account, \Dru
 /**
  * The user was built; the module may modify the structured content.
  *
- * This hook is called after the content has been assembled in a structured array
- * and may be used for doing processing which requires that the complete user
- * content structure has been built.
+ * This hook is called after the content has been assembled in a structured
+ * array and may be used for doing processing which requires that the complete
+ * user content structure has been built.
  *
  * If the module wishes to act on the rendered HTML of the user rather than the
- * structured content array, it may use this hook to add a #post_render callback.
- * Alternatively, it could also implement hook_preprocess_HOOK() for
- * user.html.twig. See drupal_render() and _theme() documentation
- * respectively for details.
+ * structured content array, it may use this hook to add a #post_render
+ * callback. Alternatively, it could also implement hook_preprocess_HOOK() for
+ * user.html.twig. See drupal_render() documentation or
+ * @link themeable Default theme implementations topic @endlink for details.
  *
  * @param array &$build
  *   A renderable array representing the user.